In the related art, a rectification Field Effect Transistor (FET), such as a Hi Side FET or a Lo Side FET, has been used in a power supply used in products such as various electronic apparatuses starting from an information processing device.
Specifically, as illustrated in FIG. 27, used is a non-insulated DC-DC (direct current to direct current) converter step-down circuit (switching control system) that includes an input power supply, an input capacitor, a smoothing inductor, a smoothing capacitor, a load, a Hi Side FET, a Lo Side FET, and an inverter. This non-insulated DC-DC converter step-down circuit turns an input current ON/OFF by a switching control signal (HiDr or LoDr) output from a Hi driver or a Lo driver, averages a voltage/current by the smoothing inductor, the smoothing capacitor, and the load, and outputs the averaged voltage/current.
In this case, the input power supply is a power supply that supplies power to the non-insulated DC-DC converter step-down circuit, and the input capacitor is a capacitor that accumulates or discharges electric energy supplied therein or therefrom by the input power supply. The smoothing inductor is an inductor that is used to perform noise suppressing, rectifying, and smoothing in the non-insulated DC-DC converter step-down circuit. The smoothing capacitor is a capacitor that charges electricity when a voltage is high, discharges the electricity when the voltage is low, and smoothes a voltage. That is, it has a function of decreasing the voltage change (ripple voltage).
Meanwhile, in the power supply (for example, non-insulated DC-DC converter step-down circuit) where the rectification FET is used, when a short circuit of the rectification FET is generated, a large current may flow through the entire circuit. As a result, the power supply and an apparatus (connected apparatus) to which the power supply is connected are failed.
Therefore, when the short circuit is generated, a protection circuit illustrated in FIG. 28 is used to protect the entire circuit. When the protection circuit detects with the use of a current sensor resistor Rsense1 an excessive current flown from the input side of the power supply, the protection circuit determines such that a short-circuit fault of the Hi Side FET is generated and opens a Breaker FET1, to separate a fault place. Likewise, when the protection circuit detects with the use of a current sensor resistor Rsense2 an excessive current flown from the output side of the power supply, the protection circuit determines such that a short-circuit fault of the Lo Side FET is generated and opens a Breaker FET2 to separate a fault place.
The protection circuit detects a generated minute voltage by the current sense resistor Rsense1, amplifies the voltage by an amplifier AMP1, and compares a voltage, which is obtained by executing filtering of ignoring a temporary peak by a delay circuit DELAY1 with a reference voltage using a comparator COM1. As a result, when the voltage obtained by executing the filtering is higher than the reference voltage, the protection circuit detects that the short-circuit fault of the Hi Side FET is generated. Likewise, the protection circuit detects a generated minute voltage by the current sense resistor Rsense2, amplifies the voltage by an amplifier AMP2, and compares a voltage, which is obtained by executing filtering of ignoring a temporary peak by a delay circuit DELAY2 with a reference voltage using a comparator COM2. As a result, when the voltage obtained by executing the filtering is higher than the reference voltage, the protection circuit detects that the short-circuit fault of the Lo Side FET is generated.
However, the related art is problematic in that the short-circuit fault cannot be accurately detected. Specifically, in the protection circuit, in the case in which the power is supplied under some conditions such as when the capacitance of the output capacitor is large and when the load rapidly changes from a light load to a heavy load, the excessive current may be generated even in a normal state. When the power is supplied under a condition where the voltage remains in the output or the load rapidly changes from the heavy load to the light load, a reverse current may be generated even in a normal state. In the protection circuit according to the related art, when the reverse current attributable to the excessive current is generated, it cannot be determined whether the reverse current is generated due to the fault. For this reason, the reverse current attributable to the excessive current that is generated in the normal state is also determined as the fault.
In the protection circuit according to the related art, the voltage and the reference voltage are compared using the comparator. However, it takes time to make the reference voltage more than the originally desired detection voltage by several tens of percents to increase a threshold margin or execute the filtering by the delay circuit DELAY to prevent erroneous detection. For this reason, a long time may be needed to detect the fault, open the Breaker FET, and separate the fault place. As a result, the input voltage of the power supply is lowered and the voltage of the entire device is also lowered. That is, there is a fear that the short-circuit fault spreads and the entire device is stopped.
In the protection circuit according to the related art, when an impedance fault is generated, a voltage effect of the current sensor resistor Rsense1 (or the current sense resistor Rsense2) is extraordinarily small or is not generated. For this reason, the impedance fault cannot be detected. As a result, heat generation and burnout are caused. In this case, the impedance fault means a state in which the FET is failed with a certain resistance value but it did not get to a perfect short-circuit state in the FEC, that is, a midway fault between a short fault and an open fault.    Patent Document: Japanese Laid-open Patent Publication No. 05-146049